1. Field of the Invention
This invention relates to a power unit which generates single-phase AC power having a commercial frequency or a like frequency, and more particularly to a power unit of this kind which uses a cycloconverter comprised of thyristors and which has a small-sized generator having a relatively low output voltage, at an input side thereof, with means for preventing overvoltage from being applied to the thyristors due to characteristics of a load connected to the power unit.
2. Prior Art
Conventionally, a so-called cycloconverter is known, which directly converts AC power with a fixed frequency to AC power with another frequency.
Such a conventional cycloconverter is normally used for converting power supplied from a commercial frequency power line or power generated by a high power generator (see Japanese Patent Publication (Kokoku) No. 60-9429, for instance), and it is generally used for driving an AC electric motor.
Further, when a small-sized generator which generates several hundred to several thousand watts is connected to the output side of such a conventional cycloconverter, a large output voltage drop can occur when a heavy load is connected to the power unit, due to the limited power-generating capacity of the generator. The output voltage drop is particularly large when a magneto generator is employed as the generator. To cope with this, thyristors having a high withstanding voltage have to be used.
To eliminate this inconvenience, the present assignee has already proposed by Japanese Laid-Open Patent Publication (Kokai) No. 10-52045 and corresponding U.S. Pat. No. 5,886,893 a power unit which is comprised of a cycloconverter formed of positive and negative converters each constituted by a two-layered structure of two half-wave converters, and which thereby reduces the voltage applied to each of thyristors of the cycloconverter to a small value.
In the proposed power unit, however, the voltage applied to each thyristor (i.e. the output voltage of the power unit) occasionally undergoes a temporary sharp rise in a peak value thereof which occurs when the power unit is in a transient operating condition, e.g. when a load connected to the power unit is turned on or off, or occurring due to deformation of the output voltage waveform when a reactor load is connected to the power unit, or the like.
To cope with this problem, countermeasures have also to be taken.